javascript - How do I get documents where a specific field exists/does not exists in Firebase Cloud Firestore?

Question

In Firebase Cloud Firestore, I have "user_goals" in collections and goals may be a predefined goal (master_id: "XXXX") or a custom goal (no "master_id" key)

In JavaScript, I need to write two functions, one to get all predefined goals and other to get all custom goals.

I have got some workaround to get custom goals by setting "master_id" as "" empty string and able to get as below:

db.collection('user_goals')
    .where('challenge_id', '==', '')  // workaround works
    .get()

Still this is not the correct way, I continued to use this for predefined goals where it has a "master_id" as below

db.collection('user_goals')
    .where('challenge_id', '<', '')  // this workaround
    .where('challenge_id', '>', '')  // is not working
    .get()

Since Firestore has no "!=" operator, I need to use "<" and ">" operator but still no success.

Question: Ignoring these workarounds, what is the preferred way to get docs by checking whether a specific field exists or does not exists?

Answer 1

Waitting for answers